The turbine outlet temperature is one parameter which can be used to control the operation of a gas turbine and for protection of a gas turbine during operation. An example for the control of a gas turbine using the turbine outlet temperatures has been disclosed for example in the EP2071157 A1.
The turbine outlet temperature can also be used for gas turbines with sequential combustion. The control of gas turbines with sequential combustion has been the object of various documents in the past. A basic operating concept for a gas turbine with sequential combustion is for example described in the EP0718470 A2.
A reliable and precise measurement of the turbine outlet temperature is a precondition for a reliable and precise control of the gas turbine operation over the whole load range.
Due to increased power generation by unsteady renewable sources like wind or solar existing gas turbine based power plants are increasingly used to balance power demand and to stabilize the grid. Thus improved operational flexibility is required. This implies that gas turbines are often operated at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures. In addition fuel from different sources with different fuel gas composition is used depending on price and availability.
At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation, during transients, as these also count for cumulative emission limits, and for different fuel compositions. To assure low emissions and stable operation an accurate and robust determination of the turbine outlet temperature is required.
Typically the arithmetic average of all turbine outlet temperature measurements is determined in a gas turbine controller and used for controlling the gas turbine operation. Theoretically, averaging of all individual temperature measurements is the best way to obtain a turbine exit temperature
However, leakages and imperfections in the gas turbine can influence the measurement. In particular cooling air leakages close to turbine outlet temperature measurement sensors can influence the locally measured turbine outlet temperature.
These leakages are typically small for a new and clean engine (or an engine reassembled after an outage) and an engine is adjusted to take into account such leakages after an outage. However, leakages and other factors impairing the temperature measurement can increase or change during operation.
These leakages affect some of the single turbine outlet temperature measurements, which are averaged to give an average turbine outlet temperature TAT AVG, because the respective temperature sensors are influenced by cold leakage flow causing a measurement error, i.e. signaling a lower temperature.
The controller then adjusts the operation to compensate for these changes in the averaged turbine outlet temperature. For example the controller can increase the fuel flow to keep the measured turbine outlet temperature (respectively the turbine inlet temperature TIT of the turbine) at the target temperature. Because of the measurement error can increase over time, actual turbine inlet temperature values increase leading to higher NOx emissions (and increased life time consumption of the machine).
For gas turbines with sequential combustion the first turbine outlet temperature measurement and the second turbine outlet temperature measurement can be influenced by leakages.